Magnetic starters. Types and device

The electromagnetic starter is used for switching powerful electricity consumers, mainly in production. This article will discuss why a magnetic starter is needed, what is the principle of operation of a magnetic starter and the design of a magnetic starter. The design and principle of the starter, both for 380V and 220V circuits, are the same for a long time and have been well developed by designers.

As already mentioned, this is a switching device, in other words, a switch, this is its purpose. Starter contacts are designed for high current flowing through heating devices and powerful electric motors. These power contacts are actuated electromagnetically, so the starters can be controlled remotely using relatively low-power circuits. Therefore, a small button or limit switch can be used to connect powerful electric motors and other loads. The reversible starter ensures that asynchronous motors are turned on in any direction - clockwise or counterclockwise, at the choice of the operator or the control system.

Principle of operation

The principle of operation of a magnetic starter actually coincides with a relay. To operate the starter from pushbuttons without latching, self-locking from contacts parallel to the button is used. To turn off, a normally closed button is used, connected in series to the control circuit. When the contacts open, the starter turns off and is ready to be turned on again immediately after the contacts of the stop button are closed.

The “push-button” version of starter control is overwhelming for manual operations. In automation circuits, starters are usually kept in the on state by a continuous signal supplied from the discrete output of the controller to the intermediate relay.

There are different types of starters, among which there are reversible magnetic starters (a “headache” for novice electricians who are trying to understand how an unusual circuit works and are not used to thinking in electrical circuits). In fact, these are two starters operating strictly alternately: if one is turned on, then the other must be turned off, otherwise there will be a short circuit between the phases.

Its principle is as follows: if in one switched-on position the sequence of phases is A, B, C, then in another position there should be, for example, A, C, B, that is, two phases should swap places. This allows you to change the direction of the rotating field in asynchronous motors and run them in different directions, either clockwise or counterclockwise.

All types of magnetic starters are united by such design elements as an alternating current electromagnet, a system of moving and fixed power and auxiliary contacts. The supporting part is a body made of heat-resistant and non-flammable plastics. These plastics must be mechanically strong and not deform at elevated temperatures. Any starter is usually three-phase.

Contact springs for smooth starting
Moving contacts (bridges)
Fixed contacts (plates)
Plastic traverse
Anchor
Starter coil
W-shaped part of the magnetic circuit
Additional contacts

The classification of magnetic starters is made according to several criteria, among which the main one is usually the size of the starter. The value does not mean the dimensions or weight of the starter, but what current it can switch and how resistant it is to an arc in circuits with inductances (when the electric motor is turned off). The basis is a non-reversible magnetic starter, since the reversible ones are assembled from the latter. Magnetic starters operate under different conditions, so they are also classified according to the degree of protection: open, protected, dust-splash-proof.

The operation of a magnetic starter very often requires a thermal relay. All types of magnetic starters have structurally compatible thermal relays. They are often produced by the same manufacturer. A particularly important application of thermal relays is to protect electric motors from overheating. The thermal relay consists of two-phase bimetallic conductors (conductors with different coefficients of thermal expansion) - one for each phase.

From an electrical point of view, they are resistors with very low resistance, and thus serve as current sensors. When too much current flows through the phases (or one of them), the bimetallic strip bends and opens the magnetic contacts, that is, the contacts in the starter coil circuit. Thermal relays are connected between the starter and the load.

Modular starters are becoming more and more common. These are DIN rail mounted starters. This is a metal profile strip fixed in cabinets on a panel. The simplicity and ease of installation are exceptional. Next to the starter (contactor) you can attach thermal relays, circuit breakers, RCDs (residual current devices), microprocessor controllers and much more. Modular devices are very easy to assemble into circuits, thanks to the wire channels laid between the DIN rails. Installation is carried out with stripped wires of the required cross-section and crimped lugs. The tips are inserted into the holes of the device terminals according to the circuit diagram and clamped with screws.

Markings required for installation and repair are applied to the top side of the starters. There is a type designation, a contact diagram and in some cases manufacturers leave space for a sticker or signature of consumer data.

Great advances in power electronics over the past decades have meant that most major manufacturers now offer consumers contactless starters containing high-power semiconductor switches. They have certain advantages. They operate silently, do not spark, and have a high switching frequency.

Some models, thanks to PWM controllers, allow smooth starting of electric motors, and even network interfaces are provided for automation. Disadvantages include high price, highly qualified repair personnel and unsafe galvanic connection to the network, which can threaten repair electricians.

Conclusion

Despite the introduction of electronic switches: already outdated thyristors and triacs, powerful field-effect transistors, and promising IGBT transistors, magnetic starters retain their importance. They are the ones who reliably break circuits, without any residual currents or leaks dangerous to personnel or equipment. In fact, this is the same immortal “switch” that is guaranteed to de-energize the electrical installation. high-quality starters never jam and you need to purchase just such ones.

The 220 V electromagnetic starter allows switching in alternating (and direct) current circuits. Typically, such devices are used when turning on powerful consumers - electric motors, heaters, etc. The need for it is justified in cases where it is necessary to frequently turn on and off the load.

Application of magnetic starters

Most often, electromagnetic starters are used to start, stop and reverse asynchronous electric motors. But since these devices are very unpretentious, they can be used for remote control of lighting, in compressor units, pumps, overhead cranes, electric ovens, conveyors, and air conditioners. The scope of application of magnetic starters is very wide. But recently, starters have been replaced by electromagnetic contactors. But, in fact, these two devices differ little in design and characteristics. Even the switching circuits are the same.

How does the starter work?

The electromagnetic contactor operates according to the following scheme:

Voltage is supplied to the working coil of the electromagnetic starter.
A magnetic field appears around this coil.
The metal core, which is located next to the coil, is pulled inward.
Power contacts are attached to the core.
When the core is retracted, the power contacts close and current flows to the load.

In the simplest case, magnetic starters are controlled using just two buttons - “Start” and “Stop”. If necessary, you can reverse it - this is done by connecting two magnetic starters using a special circuit.

How does an electromagnetic starter work?

There are two main parts to this device:

Contact block.
Directly the starter.

The contact block is installed on top of the starter housing. It is intended to expand the functionality of the control circuit. With the help of an additional block you can:

Reverse the movement of the electric motor.
Power up the lamp that signals the engine is running.
Enable additional equipment.
But the contact attachment is not always used; in most cases, one starter is sufficient.

Contact attachment

This mechanism includes two pairs of normally open and the same number of normally closed contacts. On top of the starter there are runners and hooks, and it is to them that the attachment is attached. As a result, this system is rigidly connected to the power contacts of the starter and works simultaneously with them.

Normally closed contacts by default connect elements of a circuit, while normally open contacts break them. When the magnetic starter is turned on, when the core closes the power elements, the normally closed contacts open, and the normally open contacts close.

Magnetic starter design

In general, two parts can be distinguished - upper and lower. At the top there is a group of contacts, a moving part of the electromagnet connected to the power switches, as well as an arc-extinguishing chamber. At the bottom there is a coil and a return spring, as well as the second half of the electromagnet.

Using a spring, the upper part returns to its original position after the voltage supply to the coil stops. In this case, the power contacts open. The electromagnet is assembled from W-shaped plates made of technical transformer steel. The coil is wound with copper wire, and the number of turns depends on the voltage it is designed for.

Sectors with designations

The parameters are located on the starter; there are three sectors in total:

The first indicates where a magnetic starter can be used, as well as general information about it. Namely: alternating current frequency, rated current value, conditional thermal current. For example, the designation AC-1 indicates that with the help of such mechanisms it is possible to switch the power circuits of heating elements, incandescent lamps, and other weakly inductive loads.
The second sector indicates the maximum load power that can be switched with power contacts.
The third sector usually indicates the circuit diagram of the device: it includes power and auxiliary contacts and an electromagnet coil. If there is a dotted line from the coil along all the contacts on the diagram, this means that they work synchronously.

Contact groups of starters

Power contacts are designated as follows:

1L1, 3L2, 5L3 are incoming, they are powered by AC or DC power.
2T1, 4T2, 6T3 - outgoing power contacts that connect to the load.

In fact, it doesn’t matter at all where you connect the power source and where the load. It’s just that such a scheme is generally accepted, and it must be used.

After all, if another person has to carry out repairs, he simply will not be able to immediately understand what the installer has done. The auxiliary group of contacts 13NO-14NO is designed to carry out self-recovery. In other words, this pair is used so that the start button does not have to be constantly pressed when turning on the electric motor.

Stop button

Regardless of the type of electromagnetic starter used in the design, control is carried out using two buttons - “Start” and “Stop”. Reverse may be included. The stop button is different from the others in that it is red. The normally closed contacts are mechanically connected to the button. Therefore, when the devices are operating, current flows through them unhindered.

If the button is not pressed, the metal strip, under the action of a spring, closes two contacts. If you need to stop powering the device, you just need to press the button - the contacts will open. But there is no fixation; as soon as you release the button, the contacts close again.

Therefore, to control the operation of electric motors, special circuits for switching on 220V electromagnetic starters are used. Such devices can be installed on a DIN rail without any problems, so they can be used even in the smallest mounting blocks.

Start button

It is usually green or black in color and is mechanically connected to a normally open group of contacts.

As soon as you press the start button, the circuit closes and electric current flows through the contacts. The only difference from the stop button is that by default the contacts are open. The spring holds the contact group in the open position and allows the button to be returned to its initial position after starting. This is precisely the operating principle of 220V electromagnetic starters used in control circuits for large loads.

Classic connection scheme

When implementing such a scheme, the following actions are performed:

When you press the “Start” button, the contacts close and voltage is supplied to the load.
When you press the “Stop” button, the starter contacts open and the voltage supply stops.

You can connect heating elements, electric motors, and other devices as a load. A normally open 220V electromagnetic starter can be used to turn on absolutely any load.

The power part of the circuit includes:

Contacts for connecting three phases - “A”, “B”, “C”.
Circuit breaker. It is installed between the power source and the input of the 220V 25A electromagnetic starter. The fact is that 380V is the phase-to-phase voltage, and if you measure between zero and any of the phases, it will be equal to 220V.
The load is a powerful consumer of electricity (motor, heating element).

The entire control circuit is connected to zero and phase “A”. The circuit consists of the following components:

Start and stop buttons.
Reels.
Auxiliary contact (switched on parallel to the start button).

Operation of the classical scheme

As soon as the circuit breaker turns on, three phases appear on the upper contacts of the starter, and the entire circuit is switched to standby mode. The phase under the letter “A” passes through the circuit:

Through closed contacts of the stop button.
To the contact of the start button.
To the auxiliary contact group.

In this case, the circuit is fully prepared for operation. As soon as the contacts close under the influence of the start button, voltage appears on the coil and its core is retracted. In this case, the core pulls a group of contacts along with it, closing them.

At the bottom of the magnetic starter there are power contacts, at which voltage also appears, which then goes to the electricity consumer. After releasing the start button, the power contacts will be closed due to the implementation of the “pick-up” circuit. In this case, the phase does not go through the contacts of the start button to the electromagnet, but through an auxiliary group.

Degree of protection

Devices with a degree of protection IP54 perform best. They can be used in damp and very dusty areas. You can install it in an open place without any problems. But if installation is carried out inside a cabinet, then it is enough to use devices with a degree of protection IP20. The higher the numerical index, the more severe the conditions under which the device can be operated - this applies to any electrical device. The following factors must also be taken into account:

The presence of a thermal relay, with the help of which the load is switched off when the maximum current consumption is exceeded. The use of such a device is especially important when controlling electric motors.
If there is a reverse function, then the design has two coils and six contacts. Essentially, these are a pair of starters combined in one housing.
It is imperative to take into account the wear resistance of the device, especially if the load is turned on and off by the starter very often.

Not least in the operation of any device, including a 220V electromagnetic starter, is the human factor. Unskilled workers can break the entire control chain because they do not know how to operate the equipment correctly. If the thermal protection has tripped, it cannot be switched on immediately. And you cannot restart the engine - first you need to check whether the motor is jammed or whether there is a short circuit in the power circuit.

Magnetic starters are most often used to control electric motors. Although it has other areas of application: control of lighting, heating, switching of powerful loads. They can be turned on and off either manually, using control buttons, or using automatic systems. We'll talk about connecting control buttons to a magnetic starter.

Starter control buttons

In general, you will need two buttons: one to turn it on and one to turn it off. Please note that they use contacts with different purposes to control the starter. For the “Stop” button they are normally closed, that is, if the button is not pressed, the group of contacts is closed, and opens when the button is activated. The Start button is the opposite.

These devices can either contain only a specific element needed for operation, or be universal, including one closed and one open contact. In this case, you need to choose the right one.

Manufacturers usually provide their products with symbols that make it possible to determine the purpose of a particular contact group. The stop button is usually painted red. The launcher color is traditionally black, but green is welcome, which corresponds to the “On” or “Turn on” signal. Such buttons are mainly used on cabinet doors and machine control panels.

For remote control, push-button stations are used, containing two buttons in one housing. The station is connected to the starter installation location using a control cable. It must have at least three cores, the cross-section of which may be small. The simplest working circuit of a starter with a thermal relay

Magnetic switch

Now about what you should pay attention to when examining the starter itself before connecting it. The most important thing is the voltage of the control coil, which is indicated either on it itself or nearby. If the inscription reads 220 V AC (or there is an AC icon next to 220), then a phase and a zero are required for the control circuit to operate.

Watch an interesting video about the operation of a magnetic starter below:

If it is 380 V AC (the same alternating current), then the starter will be controlled by two phases. In the process of describing the operation of the control circuit, it will become clear what the difference is.

With any other voltage values, the presence of a direct current sign or the letters DC, it will not be possible to connect the product to the network. It is intended for other circuits.

We will also need to use an additional contact of the starter, called a block contact. For most devices, it is marked with the numbers 13NO (13NO, simply 13) and 14NO (14NO, 14).

The letters NO mean “normally open”, that is, it closes only when the starter is pulled in, which can be checked with a multimeter if desired. There are starters that have normally closed additional contacts; they are not suitable for the control circuit under consideration.

Power contacts are designed to connect the load, which they control.

Their markings vary from manufacturer to manufacturer, but there are no difficulties in identifying them. So, we attach the starter to the surface or DIN rail in the place of its permanent location, lay the power and control cables, and begin the connection.

220 V starter control circuit

One wise man said: there are 44 schemes for connecting buttons to a magnetic starter, of which 3 work, and the rest do not. But there is only one correct one. Let's talk about it (see diagram below).
It is better to leave connecting the power circuits for later. This will make it easier to access the coil screws, which are always covered by the main circuit wires. To power the control circuits, we use one of the phase contacts, from which we send a conductor to one of the terminals of the “Stop” button.

This can be either a conductor or a cable core.

Two wires will go from the stop button: one to the “Start” button, the second to the block contact of the starter.

To do this, a jumper is placed between the buttons, and a cable core to the starter is added to one of them at the point where it is connected. There are also two wires from the second terminal of the “Start” button: one to the second terminal of the block contact, the second to terminal “A1” of the control coil.

When connecting buttons with a cable, the jumper is already placed on the starter, and the third core is connected to it. The second output from the coil (A2) is connected to the zero terminal. In principle, there is no difference in what order you connect the outputs of the buttons and the block contact. It is advisable to connect only the “A2” terminal of the control coil to the neutral conductor. Any electrician expects that zero potential will only be there.

Now you can connect the wires or cables of the power circuit, not forgetting that next to one of them at the input there is a wire to the control circuit. And only from this side is power supplied to the starter (traditionally - from above). Trying to connect buttons to the starter output will lead to nothing.

380V Starter Control Circuit

Everything is the same, but in order for the coil to work, the conductor from terminal “A2” must be connected not to the zero bus, but to any other phase that has not been used before. The entire circuit will operate from two phases.

Connecting a thermal relay to the starter circuit

Thermal relay is used for overload protection. Of course, it is still protected by an automatic switch, but its thermal element is not enough for this purpose. And it cannot be adjusted exactly to the rated current of the motor. The operating principle of a thermal relay is the same as in a circuit breaker.

The current passes through the heating elements; if its value exceeds the specified value, the bimetallic plate bends and switches the contacts.

This is another difference from a circuit breaker: the thermal relay itself does not turn off anything. It simply gives a signal to turn off. Which needs to be used correctly.
The power contacts of the thermal relay allow you to connect it to the starter directly, without wires. To achieve this, each product range complements each other. For example, IEK produces thermal relays for its starters, ABB produces its own. And so it is with every manufacturer. But products from different companies do not fit together.

Thermal relays can also have two independent contacts: normally closed and normally open. We will need a closed one - as in the case of the “Stop” button. Moreover, functionally it will work the same way as this button: breaking the power supply circuit of the starter coil so that it falls off.

Now you need to embed the found contacts into the control circuit. In theory this can be done almost anywhere, but traditionally it is connected after the coil.

In the case described above, this will require sending a wire from pin “A2” to the contact of the thermal relay, and from its second contact to the place where the conductor was previously connected. In the case of control from 220 V, this is the zero bus; with 380 V, this is the phase on the starter. The thermal relay is not noticeable in most models.

To return it to its original state, there is a small button on the instrument panel that resets when pressed. But this should not be done immediately, but let the relay cool down, otherwise the contacts will not engage. Before putting it into operation after installation, it is better to press the button, eliminating possible switching of the contact system during transportation due to shaking and vibration.

Another interesting video about the operation of a magnetic starter:

Checking the functionality of the circuit

In order to understand whether the circuit is assembled correctly or not, it is better not to connect the load to the starter, leaving its lower power terminals free. This way you will protect your switched equipment from unnecessary problems. We turn on the circuit breaker that supplies voltage to the object under test.

It goes without saying that it must be turned off while editing is in progress. And also, in any available way, accidental activation by unauthorized persons is prevented. If after applying voltage the starter does not turn on on its own, that’s good.

Press the “Start” button, the starter should turn on. If not, check the closed position of the “Stop” button contacts and the state of the thermal relay.

When diagnosing a malfunction, a single-pole voltage indicator helps, which can easily check the passage of a phase through the “Stop” button to the “Start” button. If, when you release the “Start” button, the starter does not lock and falls away, the block contacts are incorrectly connected.

Check - they should be connected parallel to this button. A correctly connected starter should be locked in the on position when mechanically pressing on the moving part of the magnetic circuit.

Now we check the operation of the thermal relay. Turn on the starter and carefully disconnect any wiring from the relay contacts. The starter should fall off.

Consider their connection diagrams, and most importantly, care for the devices. Today, electric motors with short-circuit rotors are widely used in industry (their share is approximately 95-96%). They work in tandem with magnetic starters. In addition, starters expand the capabilities of the electric drive. But first things first, you first need to answer the question of what purposes they are intended for.

Purpose of starters

The connection diagram of a single-phase magnetic starter allows switching of any consumer. Of course, if its power is also supplied from one phase. To be more precise, the MP allows remote control of an electric drive or other device. For example, an irreversible starter is only capable of turning on or disconnecting a consumer from the network

But reverse MPs can do not only the above. They are able to change the connection of phases to the electric motor. This means that the rotor will begin to rotate in the opposite direction. The MP is controlled using the buttons:

"Start";
"Stop";
"Reverse" (if necessary).

Moreover, these buttons have a supply voltage of no more than 24 Volts. All control is carried out using low voltage. And no more is required to power the electromagnet coil.

Types of magnetic starters

The magnetic starter, the connection diagram of which is given in the article, can be made in three versions. It all depends on the conditions under which it is used. Thus, the open version of the starters is intended for installation on a DIN rail. It goes without saying that the electrical panel must be protected from the entry of foreign objects, such as dust or liquid.

The second type of case is protected. Although it is intended for installation indoors, and not in panels, it is still unacceptable for it to get exposed to large amounts of dust, much less liquid. If it is necessary to install magnetic starters, the connection diagrams of which are given in the article, in conditions of high humidity, then it is wiser to use dust- and moisture-proof ones. True, they have a limitation - installation on the street is allowed, but only on the condition that it is not exposed to sunlight or rain.

Design of magnetic starters

Any connection diagram for which is given consists of one main part - the magnetic system. This is a coil wound around a metal core and a movable armature. All this is housed in a plastic case. But this is the basics; there are still many little things, for example a traverse that slides along the guide axes. There is an anchor on it. In addition, blocking and main contacts are connected to it. They are equipped with springs that help open when the power to the electromagnet is turned off.

How does a starter work?

The operation of MP is based on elementary physics. When you apply voltage to a winding, a magnetic field is created around the core. As a result of this, the movable armature begins to be attracted to the core. This is how any magnetic starter works, the connection diagram may only differ (depending on the presence of reverse). By the way, this can also be done using two conventional MPs. The starter contacts are normally open by default.

When the armature moves towards the core, they close. But there is another design in which the contact group is normally open by default. In this case, the picture is the opposite. Consequently, when voltage is applied to the coil, the circuit is closed and the electric drive begins to operate. But when the power to the coil is turned off, the electromagnet stops working. Return springs come into action, forcing the contact group to move to its original position.

Starter connection diagram

To begin with, it’s worth considering what a magnetic starter looks like, and the “reverse” connection diagram if used. Essentially, these are two identical devices combined in one housing. With the same success, as was said earlier, you can use simple MPs if you know the switching diagram. The starters have an interlock, which is carried out through normally closed contacts. The point is that it is unacceptable for both of them to turn on at the same time. Otherwise, a phase short circuit will occur.

There is also mechanical protection installed in the starter housing. But it can not be used if an electrical degree of protection is provided. The peculiarity of the reverse is that it is necessary to completely disconnect the electric drive from the power supply. To do this, first disconnect the electric motor from the network. After this, it is necessary for the rotor to completely stop rotating. And only after this is it possible to turn the engine in the opposite direction. Please note that the starter power must be twice that of the motor if back-switching or braking is used.

Thermal relay

Now let's look at a typical 380V magnetic starter. Its connection diagram cannot be done without additional protection. And this is a thermal relay installed on the starter housing. The main task of a thermal relay is to prevent thermal overload of the motor. They, of course, will be present, but they will be insignificant; overheating of the electric motor becomes impossible. The protection acts as a thermal overload meter, however, it is similar to the design of a circuit breaker.

The thermal relay installed on magnetic starters allows for minor adjustments. The so-called setpoint is the setting of the maximum value of current consumed by the electric motor. Typically, this adjustment is made using a screwdriver. The engine has a groove for it, as well as graduation. The procedure is simple, you just need to place the arrow on the plastic disk opposite the corresponding mark with the value of the maximum current consumption. Please note that thermal relays are not capable of short circuit protection. Use circuit breakers for this purpose.

How starters are mounted

It is worth noting that the magnetic connection diagram allows for their installation inside electrical panels. But there are requirements for all starter designs. To ensure high operational reliability, it is necessary that installation is carried out only on a perfectly flat and rigid surface. Moreover, it must be vertically located. To put it simply, on the wall of the electrical panel. If there is a thermal relay in the design, then it is necessary that the temperature difference between the MP and the electric motor be minimal.

To avoid false triggering of the starter or to protect it, it is unacceptable to install the device in places that are subject to shock, shaking, vibration, and jolts. In particular, installation on the same panel with electric starters with a current of over 150 Amperes is prohibited. When such devices are turned on and off, a sharp shock occurs. The wire connections also need to be done correctly. In order to improve contact and to prevent distortion of the spring washers of the clamps, it is necessary to bend the wires in the shape of a circle or the letter “P”.

Turning on the starter

Always try to follow electrical safety precautions and never work without turning off the power. If you have little experience, then you should always have a diagram at hand. A photo of connecting a magnetic starter is given in this article, you can read it. What needs to be done before starting the starter? The most important thing is to conduct a visual inspection for cracks, distortions, and phase short circuits. Remember that the entire drive circuit must be disconnected from power. Try to press the traverse with your hands; it should move freely along the guides. Check carefully all magnetic starters and power conductor connection diagrams in the system.

Pay attention to the connection of the starter solenoid coil. Also check that it is within the acceptable value. If 24 V is needed, then supply that much. Check all control wires to see if they are correctly connected to the Start, Stop, Reverse buttons (if necessary). Is there a lubricant solution on the contacts? If not, apply it, otherwise the lock may not work in a timely manner. After this, you can turn on the circuit and start the drive. Please note that the electric magnet coil may hum slightly in this condition.

How to care for starters

That's all, magnetic starters and connection diagrams have been completely reviewed; all that remains is to mention their care. During operation, it is necessary to constantly monitor the condition of the magnetic starter. The main maintenance work is to prevent the formation of a layer of dust, and even more so dirt, on the surface of the starter or thermal relay. From time to time, the contacts for connection to the network and to the drive must be tightened. Dust must be removed either with a rag or compressed air (not wet). It is prohibited to clean the contacts, as this will affect the service life of the device. Replacement is carried out if necessary. The service life depends on many factors, but the most important is the operating mode. If the starter is constantly in motion, making switches, then it will not last long. Its resource is measured in the number of on and off cycles, and not in hours or years.

Devices that are designed (their main purpose) to automatically turn on and off three-phase electric motors from the network, as well as their reversal, are called magnetic starters. As a rule, they are used to control asynchronous electric motors with supply voltages up to 600 V. Starters can be reversible or non-reversible. In addition, a thermal relay is often built into them to protect electrical machines from long-term overcurrent.

Magnetic starters can be produced in various designs:

Reversible;
Not reversible;
Protected type - installed in rooms where the environment does not contain a large amount of dust;
Dust-proof - installed in places where they will not be directly exposed to the sun, rain, snow (if placed outdoors, they are located under a canopy);
Open type - designed for installation in places protected from foreign objects and dust (electrical cabinets and other equipment)

Magnetic starter device

The design of the magnetic starter is quite simple. It consists of a core on which the retractor coil is placed, an armature, a plastic case, mechanical power indicators, as well as main and auxiliary block contacts.

Let's take a look at the example shown below:

When voltage is applied to the starter coil 2, the current flowing in it will attract the armature 4 to the core 1, which will result in the closure of the power contacts 3, as well as the closure (or opening, depending on the version) of the auxiliary block contacts, which in turn signal to the system controls to turn the device on or off. When the voltage is removed from the coil of the magnetic starter under the action of the return spring, the contacts will open, that is, they will return to their initial position.

The operating principle of reversible magnetic starters is the same as non-reversible ones. The difference lies in the alternation of phases that are connected to the starters (A - B - C one device, C - B - A another device). This condition is necessary to reverse the AC motor. Also, when reversing magnetic starters, the simultaneous activation of devices is blocked to avoid short circuits.

Schemes for connecting magnetic starters

One of the simplest magnetic starter connection diagrams is shown below:

The principle of operation of this circuit is quite simple: when the QF circuit breaker is closed, the power supply circuit for the magnetic starter coil is assembled. The PU fuse provides short circuit protection for the control circuit. Under normal conditions, the thermal relay contact P is closed. So, to start the asynchronous machine, we press the “Start” button, the circuit closes, current begins to flow through the coil of the magnetic starter KM, the core is retracted, thereby closing the power contacts of the KM, as well as the block contact BC. The block contact BC is needed in order to close the control circuit, since the button, after it is released, will return to its original position. To stop this electric motor, just press the “Stop” button, which will disassemble the control circuit.

In case of prolonged current overload, the thermal sensor P will be triggered, which will open contact P, and this will also lead to the machine stopping.

When using the connection diagram above, you should take into account the nominal voltage of the coil. If the coil voltage is 220 V, and the motor (when connected in a star) is 380 V, then this circuit cannot be used, but can be used with a neutral conductor, and if the motor windings are connected by a delta (220 V), then this system is quite viable.

Circuit with neutral conductor:

The only difference between these connection schemes is that in the first case, the control system power is connected to two phases, and in the second to a phase and a neutral conductor. When automatically controlling the starting system, instead of the “Start” button, a contact from the control system may turn on.

You can see how to connect a non-reversible magnetic starting device here:

The reversible connection circuit is shown below:

This circuit is more complex than when connecting a non-reversible device. Let's look at the principle of its operation. When you press the “Forward” button, all the actions described above occur, but as you can see from the diagram, a normally closed contact KM2 appears in front of the forward button. This is necessary to electrically block the simultaneous activation of two devices (avoiding short circuits). When you press the “Back” button while the electric drive is running, nothing will happen, since the KM1 contact in front of the “Back” button will be open. To reverse the machine, you must press the “Stop” button, and only after turning off one device can you turn on the second.

And a video of connecting the reversible magnetic starting device:

When installing magnetic starting devices with thermal relays, it is necessary to install with a minimum difference in ambient temperatures between the electric motor and the magnetic starting device.

It is undesirable to install magnetic devices in places subject to strong shocks or vibrations, as well as near powerful electromagnetic devices whose currents exceed 150 A, since they create quite large shocks and jolts when triggered.

For normal operation of the thermal relay, the ambient temperature should not exceed 40 0 C. It is also not recommended to install it near heating elements (rheostats) and not install them in the most heated parts of the cabinet, for example at the top of the cabinet.

Comparison of magnetic and hybrid starters: